Tin plate and method of producing



United States Patent 3,074,154 TlN PLATE AND METHOD OF PRODUCING VincentP. Pearson, Chesterton, Ind., and James E. Mc-

Farland and Henry M. Roelofs, Chicago, Ill., assignors to Inland SteelCompany, Chicago, 111., a corporation of Delaware N0 Drawing. Filed Nov.2, 1959, Ser. No. 850,118 17 Claims. (Cl. 29--196.4)

This invention relates generally to tin plate and more particularly toan improved tin plating process which includes the step of providing atleast a portion of the surface of a metal strip with a coating which isadapted to serve as an identification marking or protective layer whichretains a high degree of corrosion resistance, and to the improved tinplate thus produced.

In general, the production of tin plate by the electrolytic tinningmethod involves the steps of passing a cold reduced, annealed ferrousmetal sheet or strip, such as black plate, continuously through a tankcontaining an acidic or alkaline solution of a tin salt and thereinelectrodepositing tip on the strip. After leaving "the tinning tank, therough, freshly plated strip is passed through a furnace or oven at atemperature sufficiently high to fuse the tin in order to form a smooth,bright tin coating on the strip. A relatively recent development in theforegoing general electrolytic tinning process is the production ofdifferentially coated tin plate wherein the electrolytic tinning step isso regulated that one side of the metal strip receives a relativelylower weight coating of tin. For purposes of tin can manufacturing andthe like, a relatively heavy corrosion resistant coating of tin isrequired only on the inside of the can. Thus, the use of differentiallycoated tin plate avoids an unnecessarily heavy external coating of tinon the outside of the can, thereby resulting in important economies inthe use of tin.

With the advent of differentially coated tin plate in the industry, itwas necessary that the tin plate manufacturers have some means formarking the differentially coated tin plate which would permit the canmanufacturer to identify the side having the lighter tin coatingthereon, or otherwise distinguish between the different sides of thetip-plated strip. As the differentially coated tin plate is dischargedfrom the fusion step of the electrolytic tinning line, how ever, bothsides of the strip normally possess the same customary bright, highlyreflective finish.

Various marking techniques have been proposed and tried heretofore butno entirely satisfactory scheme has heretofore been developed fordistinguishing one side of the plated strip. For example, certain typesof printed markings have been tried but these have not been acceptablebecause the marking or design tended to fade after a period of storage.Another method of identification which has been utilized to some extentinvolves the use of a roughened roll at the skin mill so that the steelstrip prior to introduction to the electroplating bath has one side or aportion thereof roughened. The roughened surface after electrolytictinning possesses a dull surface appearance or a decreased reflectance,as compared with the tin coating on the smooth side of the strip.Although this purely mechanical pretreatment of the strip to effectdulling of one side gives a permanent marking, it has been found thatthe dulling eifect is far from uniform. Furthermore, the corrosionresistance of the tin plate is adversely affected and this, of course,is a serious objection for most uses of tin plate. Moreover, the tinplate producer also finds the technique'inconvenient, since itnecessitates frequent changes of rolls at the skin mill due to thetendency of the rough roll to become smooth after a relatively shortperiod of rolling time.

Another marking method which'has been utilized to some extent, involvestreating one side of the steel strip anodically between the eelctrolytictinning and fusion steps. For satisfactory dulling by the latter means,however, it is also necessary that the base metal strip be roughened atleast slightly in the rolling operation prior to electrolytic tinning,thereby introducing the previous objections of non-uniformity of dullingand reduced corrosion resistance of the tin plate. Moreover, the anodictreatment frequently is ineffective along the marginal edges of thestrip so that the over-all dull surface appearance desired by the canmanufacturers is not obtained.

Still another method of marking or dulling tin plate, which avoidscertain of the objectionable features of the preceding methods,comprises applying an alkali metal salt as a continuous coating or in adistinctive pattern, to one side of a tin coated strip prior to thefusion heating thereof. However, since the latter process requires theuse of a chemical salt on the exposed surface of the tin plate, it ishighly preferable to provide a suitable process of marking tin platewhich avoids applying a chemical to the exposed surface of tin plate.

Accordingly, it is a primary object of the present invention to providenovel and improved mean for marking or dulling tin plate.

Another object of the invention is to provide a novel and improved meansfor providing an identification marking on one side of differentiallycoated tin plate.

A further object of the invention is to provide a novel and highlysatisfactory means for marking or dulling tin plate which avoidsappreciable impairment of the corrosion resistance properties of the tinplate.

It is another object of the present invention to provide an improvedelectroplated tin coated strip having a marked or dulled tin coating.

Still another object of the invention is to provide an electroplated tincoated strip having high corrosion re sistance. Another object of theinvention is to provide a novel step in an electrolytic tinning processfor imparting a uniformly dull surface appearance to one side of the tinplated strip.

Other objects of the invention will be apparent to those skilled in theart from the accompanying description and claims to follow.

In the usual electrolytic tinning processes adapted to makedifferentially coated tin plate, a steel strip is formed by hot-rollingfrom slabs, continuously pickling the strip, cold-reducing to therequired thickness, cleaning, annealing, temper or skin rolling, andpassing the strip through electrolytic cells where the desired coatingof tin is electrodeposited on each side of the strip, after which thetin coatings are fused in a fusion unit to provide a bright,lustrous-appearing surface. The present invention comprises animprovement in the foregoing electroplating processes wherein there isapplied to the free metallic surface of the base steel strip, a solutionor suspension of a reducible compound of a metal such as molybdenum,nickel, cobalt, titanium, tungsten, and antimony, said metals generallybeing in the group of adherence-promoting metals, and in addition,compounds of such metals as bismuth, tellurium, and arsenic, all thesaid compounds being reducible to the free metal at temperatures belowabout 2000 F. The selected compound preferably is applied to the metalsurface just prior to its entering'the preheat zone of the continuousannealer, or just before recoiling after cleaning and drying thestripfor box annealing. The strip coated with the metal compound is thenheated under conditions to cause the said metal compound to be reducedto a free metal which difiuses into the steel strip to provide a surfacecoating capable of withstanding skin rolling, and which gives, afterelectroplating and fusion, a much less reflectant tin plated surfacethan the normal fused tin plated surface. Also, where high corrosionresistance is desired, a compound of a metal of the above group can beselected which renders the tin plated surface more highly resistant tocorrosion than previous tin plated surfaces, such as ti tanium andnickel.

The reagents which can be used for marking steel plate for use in themanufacture of electroplated tin plate in accordance with the presentinvention comprise aqueous solutions of water soluble compounds of themetals including molybdenum, nickel, cobalt, titanium, antimony,tungsten, bismuth, tellurium and arsenic, or aqueous sus-. pensions ofcompounds thereof, which on being subjected to heat in a neutral orreducing atmosphere, are reducible to a free metal and diffusible intothe steel strip. The compounds of the above metals which are most usefulin the present invention are the non-volatile metal oxides which arereadily reducible to the free metal on heating in a non-oxidizingatmosphere. It is, of course, advisable to avoid using compounds of thesaid metals which on heating under neutral or reducing conditionssublime at low temperatures or leave a residue on the steel strip inaddition to the free metal, particularly where the said residue is notreadily removable by a washing or pickling treatment following theannealing step. More particularly, the specific compounds which can beused for marking or coating in the present invention include molybdenumoxide (Mio nickel oxide (NiO), cobalt oxide (C), titanium oxide (TiOantimony oxide ($13203), tungsten oxide (W0 or bismuth oxide 121 0,

It is also within the scope of the present invention to provide an oxidecoating on the metal strip by means of a solution or suspension of acompound or salt of the said metals which can be decomposed or oxidizedto yield the desired oxide in 'situ. For example, ammonium molybdate ormolybdic acid, can be deposited on the metal strip and readilydecomposed or oxidized by heating, to yield molybdenum trioxide.Similarly, ammonium tungstate, tungstic acid, and nickel formate, aretypical examples of compounds of the above metals which can be used toform the corresponding metal oxide in situ. Among the compounds whichdecompose to the oxide and which are useful in the present invention asmarking or coating agents, are the formates, oxalates, nitrates,tartrates, and the ammonium salts of certain of the said metals. Also,it is possible to use a combination of any twoor moreof the said salts,such as molybdenum oxide and nickel oxide in an. admixture thereof, or acomplex salt of two or more of the said metals, such as themolybdenum-nickel complex salt formed by mixing a solution, of ammoniummolybdate and nickel chloride, or nickel nitrate.

The foregoing marking or coating agents of they present invention areapplied to the surface of the metal strip in the form of solutions,suspensions, or dispersions. One

method of providing the oxide coating on the iron or steel base is toform' a suspension or dispersion in water or. other suitable liquidmedium of an oxide or oxides, in finely divided or powdered form andthen apply the liquid coating material to the surface of the base byanyconvenient technique, such as spraying, brushing, wiping,roll-coating, dipping, electrostatic fogging or the like. Usually, waterwill be the most convenient liquid medium but other liquid vehicles suchas glycerine, light oils,- or-the like, can be used. In the case of anaqueous medium, it is usually advantageous to employ therewithanaddeddispersing or suspending agent, such as starch,

and preferably including a surface active agent, in order to form arelatively stable suspension or dispersion of the finely divided oxidein the aqueous liquid. We have found that methyl cellulose, corn syrup,cornstarch, or other thickening agent is highly satisfactory for thispurpose.

When the coating is applied as a suspension or dispersion containingstarch as a suspending agent, it is preferred to heat or bake thecoating to polymerize the starch before heating to the elevatedtemperature required for reduction of the oxide by interaction with themetallic iron. Thus, heating for a few minutes at a temperature betweenF. and 300 F. polymerizes the starch and forms a firmly adherent coatingof the oxide on the base before reduction thereof to the metal state anddiffusion into the base.

Although any of the above-mentioned methods of applying the markingsolution or suspension may be used, spray application is generally themost satisfactory for large-scale commercial operation, since therelatively large quantities of solution or suspension required on thesheet can readily be obtained by the use of sprays. Ordinary mechanicalsprays or atomizers can be utilized, but the preferred method ofapplication is by electrostatic coating because of the ease with whichover-spray is controlled, thereby avoiding contamination of the workingarea. Moreover, it is extremely important in the case of differentiallycoated tin plate that the application of the solution or suspension beconfined to only one side of the strip. With electrostatic coating, it,is a relatively simple matter to avoid wrap around, i.e., contact of themarking reagent with the reverse side of the strip relative to. thespray nozzles. Electrostatic coating is Well known in the art and neednot be described in any detail except to say that the treating solutionis converted into a spray by air pressure or mechanical means under theinfluence of an electrostatic field. so that the spray particles areelectrically charged and are attracted to the strip and depositedthereon. Reference is made to such prior art patents asRansburg et al.2,334,648, Ransburg 2,658,009, and'Ransburg 2,698,814 for variousdetails of the electristatic coating methods and devices.

In order to reduce the surface tension of the aqueous treating solution.or suspensions and to insure uniform distribution of the solution:asapplied to the surface of the strip, it is. preferred to include inthetreating solution or suspension a small amount, inthe order of fromabout 0.01% to about 0.5% by volume, of a suitable surface: active orwetting agent. It has been found that the non-- ionic surface active.agents, are particularly effective for" this, purpose, e.g-., thepolyoxyethylene ethers and the polyethylene ethers and the polyethyleneglycol alkyl ethers. Any tendency to dewetting detracts from the desireduniformv dulling or marking of the surface, and the presence of asurface active agent minimizes-this. effect to a large extent;

In order to effect uniformdulling of a tinelectroplated surface inaccordance with the. present invention, it ispreferred that the metalcompound: be present thereon to. the extent sufiicient to provide about0.1 gram of the metal per square foot of treated surface. An oxidecoating of the said metals, such as molybdenum oxide or nickel oxide,having a thickness of about 0.000005 inch will provide the optimumrequired amount of metal. In order to effect an increase in thecorrosion resistance of tin plate over and above the normal resistanceattributable to, the tin, it is. preferred that the metal compound bepresent on the surface of the steel strip in an amount suflicient toprovide about 1 gram of metal per square foot of treated surface. Anoxide coating of the herein disclosed metals, such as nickel oxide,having a thickness of from 0.001 and 0.00005 inch, will provideincreased corrosion resistance. 'The amount of compound placed on thesurface of the base plate is always correlated with the time andtemperature of. treatment so that the compound is completely reduced anddiffused into the base within the limited time permitted in the heatingstep employed. From a practical consideration, the range can vary fromabout 50 to about 500 mg. per square foot of surface, depending upont heparticular metal salt employed and the degree of dullness desired. Theconcentration of the aqueous treating solution or suspension is notparticularly important as long as the aforementioned minimum amounts ofmetal salt are deposited on the surface of the steel strip. Thesolutions or suspensions of the said metal compound can, for example,have a concentration from about 100 to about 360 grams per liter ofsolution. Also, the starch can be used in an amount between about to 30grams per liter. It will be understood that the quantity of the metalcompound applied to a given area of the steel strip must be correlatedwith the concentration of the metal compound therein so as to obtain therequired minimum deposit of the said metal compound per square foot oftreated surface. Actually, of course, the maximum concentration of themetal salt in an aqueous solution is determined only by the watersolubility of the particular metal salt employed.

Following the application of the metal compound on the steel strip, thestrip is heated to an elevated temperature, preferably by passing thestrip through the conventional continuous annealing furnace, wherein itis maintained in the non oxidizing atmosphere therein for a suflicienttime to effect decomposition of the salt to the oxide, if the oxideitself is not employed, and thereafter complete the reduction thereof tothe free metal, after which the metal is diffused into the surface ofthe steel strip. Broadly, the temperature range at which thereduction-dilfusion heating step takes place is from about 1150 F. toabout 1400 F., depending upon the particular metal compound employed andthe time allowed for the heating step. For example, when molybdenumoxide is employed, a temperature of about 1400 F. is maintained and thestrip is allowed to remain in the continuous annealing furnace for aperiod of about 2 to 10 seconds. In a box annealing process, wherein themolybdenum oxide is deposited on amild steel base (0.05 to 0.1% carbon),the heating step is conducted at a relatively lower temperature rangebetween about 1150 F. to about 1200 F., and the period of heating is inthe order of from 3 to 12 hours. It will be understood that the time andtemperature of the heating step are correlative factors so thatsubstantially equivalent results can be obtained at conditions of hightemperature-short time, or low temperature-long time.

While it is possible to carry out the heating and reducing steps in avariety of treating atmospheres, the heating of the oxide coating shouldbe carried out in an atmosphere substantially non-oxidizing to iron, andpreferably in an oxygen-free atmosphere, either neutral or reducing. Awide variety of atmospheres which are either neutral or reducing to apredetermined degree are well known in connection with the variousnormalizing, annealing, and heat treating operations which arefrequently carried out in the steelmaking art. However, generallyspeaking, such atmospheres comprise specially prepared mixtures of inertgases containing none or a certain predetermined amount of hydrogenwhich determines the reducing character of the atmosphere.

It is generally preferred to apply the marking solution of the presentinvention to the light-weight side of the strip which ordinarily becomesthe exterior of the metal can or container. It is also advisable topretreat the surface of the steel strip before applying the marking orcoating solution so as to remove all soil and oil from the surfacethereof by such means as vapor degreasing, or by means of the alkalinecleaner which is a standard operation in the continuous annealingprocess.

To illustrate the present invention, the following specific film oxide.

examples are set forth without, however, limiting the ditions described:

EXAMPLE 1 Clean samples of rolled steel strips measuring approximately3" x 8", were spray coated to completely wet one surface thereof withone of the test solutions containing a compound of the presentinvention, and thereafter heated under conditions simulating those of acontinuous annealing furnace (i.e., non-oxidizing atmosphere and atemperature of at least about 1250 F.). After allowing the steel stripsto remain in the laboratory furnace under the foregoing conditions forabout 10 seconds, wherein the compound of the test solution is reducedto a free metal and wherein the free metal is diffused into the steelstrip, the test strips were electrolytically tinned so as to provideone-quarter pound per base box tin coating on the treated surface, andthereafter the tin coating was fused by heating to a temperature ofabout 450 F. to provide a smooth tin coating. The test steel strips andcontrol samples were examined to determine the relative appearance andreflectance of the tin plate coating formed thereon.

The two test aqueous solutions employed in the fore going testscomprised:

Grams per liter aqueous solution (1) Ammonium molybdate (2) Molybdenumoxide EXAMPLE 2 Differentially coated tin plate was prepared inaccordance with the present invention on production equipment, whereinthe test solution was applied to the top surface of a coil of blackplate immediately prior to the entry of the plate into the preheatfurnace and annealing furnace. The solution was applied by contactingthe upper surface of the strip with a wiping swab saturated with thetest solution. Thereafter, the coil was passed through the normalannealing cycle, skin mill, and electrolytic tinning process, wherein itwas differentially coated by applying one-quarter pound and one poundtin coatings per base box on the upper and lower surfaces thereof,respectively. The electroplated steel strip was then fused in the normalmanner to provide a smooth t1n plate surface. The tes't'solution had thefollowing composition:

Nickel 'formate 250 grams per liter in 1:1 mixture of corn syrup andwater by volume. Nonionic wetting agent (Kyro E0) -0.1% by volume.

On heating, the nickel formate decomposes to provide a uniform coatingof the nickel oxide which on continued heating in a non-oxidizingatmosphere is reduced to the free nickel metal and is difiused into thesurface of the black plate.

Tests were made on the lightweight tin plate side (Lot A) and on controlsamples (Lot B) tov determine the coating weight, solderability, lacqueradhesion, pickle lag, stain, chromium residue, corrosion resistance andThe test data obtained appear in the following Table 1:

'Nonionic Table 1 Total Pickle No. of Film Oxide, Lot Tin Sclder-Fabrication Lag, Stain Cr. hrs for M coulb. 13.3. ability Lacquer Sec.Res. 25% Corlornbs/in.

rosion A 12 11-23 OK OK OK 6 K .05 85 i 12 B-. 12 24-23 OK OK OK 8 OK.06 100 11 It will be evident from the foregoing, that identificationmarking of dual-coated tin plate can be accomplished by treating thesurface of the steel strip prior to continuous annealing withoutappreciably impairing the desired properties of tin plate, such as goodsolderability and lacquer adherence and corrosion resistance.

EXAMPLE 3 Differentially coated tin plate was produced on fullvscaleproduction equipment as in Example 1, wherein the treating solutionconsisted, of:

Ammonium molybdate 200 grams per liter in 1:1

mixture of corn syrup and water by volume.

wetting agent (Kyro E0) 0.1% by volume.

The electrolytically coated tin plate was observed to have a veryuniform dull appearance in contrast with the bright appearance of thecontrol sample of normal electrolytic tin plate which was not treatedwith the ammonium molybdate solution.

It is evident from the foregoing description, that while the inventionis clearly applicable to a batch of semicon-tinuous tinning operation,it is particularly adapted for use in a continuous annealing line, priorto electrotinning in which case the marking or protective coating agentis conveniently applied by means of a strategically located dip-coatingbath, brush applicator, or spray, positioned immediately before theannealing furnace. A marking or protective coating procedure asdescribed herein, is useful in any application wherein a protective ordull tin-plated surface is desired, but is particularly useful inmarking one side only of differentially coated tin plate with a readilydistinguishable symbol or pattern, in order to enable one of theelectrolytically tin plated surfaces to. be readily distinguished fromthe other tinplated surface, or to enable one manufacturers tin plate tobe readily distinguished from another manufacturers tin plate.

We claim:

1. In the production of ditferentially coated tin plate byelectrolytically depositing tin on a base steel sheet and. thereafterheating said tin to an elevated temperature for fusingtheelectrolytically deposited tin, the improvement in the method of markingone side of said tin plate comprising: providing on at least certainportions of one side of said sheet having a free metallic surface aliquid film coating containing an oxygen compound of a metal selectedfrom a group consisting of molybdenum, cobalt, nickel, titanium,tungsten, antimony, and bismuth, said compound being heat reducible tothe free metal, heating said coating of said compound in a non-oxidizingatmosphere to reduce the said compound to the free metal and continuingsaid heating to diffuse the said metal substantially completely into thesurface of the base steel sheet, electrolytically depositing tin overthe said surface of the steel sheet and heating the sheet to a tinfusion temperature to form a smooth dull tin surface on said certainportions, whereby said one side of. said tin plate is distinctivelymarked.

, 2. A process as in claim 1, wherein the said metal sheet isheatecl. inthe. first mentioned heating step at a temperature between about 1150 F.and 1400 F.

3. A process as in claim 1, wherein the said oxygen compound is selectedfrom a group consisting of oxides and oxygen compounds decomposable toyield said oxides.

4. A process as in claim 1, wherein the said oxygen compound ismolybdenum trioxide.

5. A process as in claim 1, wherein the said oxygen compound is ammoniummolybdate.

6. A process as in claim 1, wherein the said oxygen compound is nickeloxide.

7. A process as in claim 1, wherein the said oxygen compound is nickelformats;

8. In the production of differentially coated tin plate byelectrolytically depositing tin on a base steel sheet and thereafterheating the tin coated sheet to an elevated temperature for fusing thetin thereon, the improvement in the method of marking one side of thetin plate comprising; providing on said one side of the base steel sheeta fiuid coating of a molybdenum oxide, heating the said oxide coating ina substantially non-oxidizing atmosphere sufficiently to reduce the saidoxide to the free metal and to substantially completely diffuse themetal into the surface of the said base metal sheet providing a surfacesubstantially devoid of said oxide and said free metal, thereafterelectrolytically depositing tin over said diffused metal surface of thesaid sheet, and fusing the electrolytically deposited tin by heating toa tin fusion temperature to provide a smooth dull tin surface on thesaid base sheet.

9. In the production of dilferentially coated tin plate byelectrolytically depositing tin on a base steel sheet and thereafterheating the tin coated sheet to an elevated temperature for fusing thetin thereon, the improvement in the method of marking one side of thetin plate comprising; providing on said one surface of the base steelsheet a fluid coating of nickel oxide, heating the said oxide coating in.a substantially non-oxidizing atmosphere sufficiently to reduce thesaid oxide to the free metal and to substantially completely diffuse themetal into the surface of the said base metal sheet providing a surfacesubstantially devoid of said oxide and free metal, thereafterelectrolytically depositing tin over said diffused metal surface of thesaid sheet, and fusing the electrolytically deposited tin by heating toa tin fusion temperature to provide a smooth dull tin surface on thesaid base sheet.

10. In the continuous production of differentially coated tin plate bypassing a continuous base metal strip through an annealing zone andthereafter through an electrolytic tinning zone to efiect controlledelectrolytic deposition of tin so that the opposite sides of the striphave relatively light and relatively heavy tin coatings, respectively,and thereafter continuously passing the differentially coated stripthrough a fusion zone at elevated temperature for fusing theelectrolytically deposited tin, the method of providing an identifyingmarking on one side of the strip which comprises: continuously passingthe said strip through a treating zone and therein providing on only oneof the sides of the said strip a uniform fluid coating over at least aportion of the surface thereof of an oxygen compound of a metal selectedfrom a group consisting of molybdenum, cobalt, nickel, titanium,tungsten, antimony, and bismuth, said compound being reducible to a freemetal when heated in a non-oxidizing atmosphere at a temperature betweenabout ll50 F. and 1400 F., heating the said compound continuously in anannealing zone having a non-oxidizing atmosphere at a temperaturebetween about 1150" F. and 1400 F. to reduce the said compound to thefree metal and substantially completely diffuse the said metal into thesurface of the said strip until said surface is substantially devoid ofSaid compound and said free metal, thereafter electrolyticallydepositing tin on the said one side of said strip which is substantiallydevoid of said compound and said free metal to provide a relativelylight tin coating on the side of the strip and a relatively heavy tincoating on the other side thereof, and thereafter continuously heatingthe tin coatings to a fusion temperature to provide smooth tin surfaces;whereby the light tin coating side is provided with a dull surfacemarking having a materially reduced reflectance as compared with thebright reflectant surface of the heavy tin coating side.

11. A process as in claim 10, wherein the said compound is molybdenumoxide.

12. A process as in claim 10, wherein the said compound is ammoniummolybdate.

13. A process as in claim 10, wherein the said compound is nickel oxide.

14. A process as in claim 10, wherein the said compound is nickelformate.

15. A differentially marked tin plated ferrous metal sheet havingthereon electrolytically deposited tin coatings which comprises, aferrous metal sheet having on at least certain portions of only one sideof said sheet a quantity of substantially completely reduced metalselected from the group consisting of molybdenum, cobalt, nickel,titanium, tungsten, antimony, and bismuth which is completely diffusedinto the ferrous metal surface forming a thin diffused surface layersubstantially devoid of metal oxides and unalloyed metals, and said oneside of said sheet and the opposite side of said sheet both having anelectrolytically deposited tin coating thereover with the tin coatingformed over said thin diffused surface layer being fused and providing adull appearance suitable for distinctively marking said one side of saidtin plated metal sheet.

16. A ferrous metal sheet as in claim 15 wherein the said thin layer ofreduced metal provides from about 50 to about 500 mg. metal per squarefoot of surface coated by said thin layer.

17. A ferrous metal sheet as in claim 15 wherein said thin layerprovides about one gram of free metal per square foot of surface coatedby said thin layer.

References Cited in the file of this patent UNITED STATES PATENTS2,266,330 Nachtman Dec. 16, 1941 2,303,035 Fink Nov. 24, 1942 2,381,778Schoonmaker et al. Aug. 7, 1945 2,621,988 Donley Dec. 16, 1952 2,794,775Buckingham June 4, 1957 2,839,437 Manko June 17, 1958 2,876,176 Pearsonet a1. Mar. 3, 1959

1. IN THE PRODUCTION OF DIFFERENTIALLY COATED TIN PLATE BYELECTROLYTICALLY DEPOSITING TIN ON A BASE STEEL SHEET AND THEREAFTERHEATING SAID TIN TO AN EVALUATED TEMPERATURE FOR FUSING THEELECTROLYTICALLY DIPOSITED TIN, THE IMPROVEMENT IN THE METHOD OF MARKINGOF ONE SIDE SAID TIN PLATE COMPRISING: PROVIDING ON AT LEAST CERTAINPORTIONS OF ONE SIDE OF SAID SHEET HAVING A FREE METALLIC SURFACE ALIQUID FILM COATINH CONTAINING AN OXYGEN COMPOUND OF A METAL SELECTEDFROM A GROUP CONSISTING OF MOLYBDENUM, COBALT, NICKEL, TITANIUM,TUNGSTEN, ANTIMONY, AND BISMUTH, SAID COMPOUND BEING HEAT REDUCIBLE TOTHE FREE METAL, HEATING SAID COATING OF SAID COMPOUND IN A NON-OXIDIZINGATMOSPHERE TO REDUCE THE SAID COMPOUND TO THE FREE METAL AND CONTINUINGSAID HEATING TO DIFFUSE THE SAID METAL SUBSTANTIALLY COMPLETELY INTO THESURFACE OF THE BASE STEEL SHEET, ELECTROLYTICALLY DEPOSITING TIN OVERTHE SAID SURFACE OF THE STEEL SHEET AND HEATING THE SHEET TO A TINFUSION TEMPERATURE TO FORM A SMOOTH DULL TIN SURFACE ON SAID CERTAINPORTIONS, WHEREBY SAID ONE SIDE OF SAID TIN PLATE IS DISTINCTIVELYMARKED.